alternator-test: add a table with string sort key
alternator-test: add a table with string sort key

String sort key will be needed to ensure that empty string keys
are not accepted.

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla